1. Field of the Invention
The preset invention relates to a multi-joint robot having a gas spring, and in particular, relates to a technique for estimating the amount of decrease in the inner pressure of the gas spring.
2. Description of the Related Art
A gas spring has a piston structure in which a compressible inert gas (hereinafter merely referred to as “gas”) is enclosed, and is capable of generating reactive force by compressing the gas due to the motion of a piston rod relative to a cylinder. Generally, such a gas spring is small and lightweight in comparison to a coil spring having the same degree of output as the gas spring, and thus the gas spring is useful for reducing the size and weight of an entire system.
In a multi-joint robot having a gravitative axis, as an apparatus for supplementing the power (or torque) of a servomotor for driving the axis, an apparatus (hereinafter referred to as a balancer), capable of generating a force in a direction opposite to the direction of a load due to gravity, may be utilized. When the gas spring is applied to the balancer (such a balancer is referred to as a gas balancer), since the output of the gas spring is relatively high in spite of its size and weight, the gas spring can generate relatively high auxiliary torque with a compact mechanism. Therefore, by virtue of the gas spring, a compact and lightweight robot capable of conveying a relatively heavy load may be constituted.
For example, JP H10-138189 A discloses a balancer for reducing the load moment of a vertical rotation axis 2 which drives a rotating arm 3 on a base 1. The balancer has a cylinder 4, a rod 5 and a piston 6, wherein an air-compressing chamber 10 is connected to cylinder 4 via a communication part 11, and gas such as air (compressible fluid) and viscous liquid such as oil are enclosed in cylinder 4 and chamber 10.
On the other hand, in the gas spring, it is known that gas enclosed in the cylinder may leak to the outside the cylinder due to the motion of the piston rod relative to the cylinder, whereby the inner pressure of the cylinder gradually decreases. Therefore, the force generated by the balancer is lowered, resulting in an alarm informing of a lack of torque being output, thereby causing a robot to be brought to an emergency stop.
As means for avoiding such a problem due to a reduction of the inner pressure of the cylinder, several techniques have been proposed. For example, JP 2007-298513 A discloses a circuit pressure display device capable of displaying the reduction of inner pressure of a gas spring circuit. JP 2009-270987 A discloses a pressure indicator for a gas spring, configured to indicate the pressure of compressible gas contained in a gas filling chamber of the gas spring.
Further, JP 2012-519083 A discloses a balancing device including a gas spring 4 and a compression system 5. Compression system 5 has a compressor 6 and an automatic control system 7 for adjusting the pressure within gas spring 4, wherein automatic control system 7 controls a pressure control valve 9 for limiting the flow rate of gas from or to gas spring 4, based on a signal from a pressure sensor 8 for detecting the pressure in gas spring 4.
As described in JP 2007-298513 A or JP 2009-270987 A, when the pressure indicator is attached to the outside of the gas spring, the cost of equipment is increased, based on the cost of the pressure indicator. It is necessary to arrange a connecting means such as a connector between the pressure indicator and the gas spring. However, use of the connecting means increases the possibility of gas leakage. Further, gas leakage may occur due to breakage of the pressure indicator.
As described in JP 2012-519083 A, when the pressure sensor and the pressure control valve are arranged in the gas spring, the cost of equipment is increased, based at least on the cost of the sensor and the valve, and the possibility of gas leakage increases, similarly to JP 2007-298513 A or JP 2009-270987 A. Further, since a device such as a gas tank 11 connected to pressure control valve 9 is necessary, peripheral equipment may become larger in size.